The direct harvesting, conversion, and storage of solar energy in chemical bonds is a promising approach for meeting the global demand for clean energy. Efficient sunlight driven water splitting remains one of the most desirable ways to store energy by forming energy dense H2. See Fujishima & Honda, Nature, 1972, 238, 38-39. The splitting of H2O consists of two half reactions: water oxidation to form O2 and water reduction to form H2.
It is frequently straightforward experimentally to increase the thermodynamic driving force for a catalyzed process by increasing the external bias (potential) on the working electrode or use of a more potent reagent in chemical reactions. At some point increasing the additional driving force will overcome the activation energy of the rate-limiting step and reaction will proceed. For electrochemical processes, this additional driving force is termed the overpotential. Catalysis is most important and most relevant when the rate accelerations are seen under conditions with little or no externally applied driving force. Thus, there is a need for oxidation catalysts (WOCs) and other multi-electron-transfer catalysts that are effective at minimal overpotentials.
Carbon ligand based WOCs are prone to degradation in the presence of O2 in the air where catalysts succumb to inactivation by ligand oxidation. Coordination compound WOCs are also susceptible to hydrolysis. Polyoxometalates POM systems are distinct and attractive water oxidation catalyst (WOC) because they are water soluble, oxidatively resistant, and hydrolytically stable in certain pH ranges. See Lv et al., Chem. Soc. Rev., 2012, 41, 7572-7589.
Yin et al. report soluble water oxidation catalyst with polytungstate ligands. See Science, 2010, 328 (5976): 342-345. Stacke and Finke report electrocatalytic water oxidation with a heterogeneous CoOx catalyst derived from polyoxometalates. Li et al. report the synthesis, crystal structure, and properties of two sandwich-type tungstovanadates. See Inorganica Chimica Acta, 2009, 362:2796-2801. See also WO2010/107919.
References cited herein are not an admission of prior art.